Use of LED Indicator lamps in Vehicles
Posted: Mon 22 Jun, 2009 10:04 am
Just a little background info on the substitution of LED lamps for indicators in vehicles - have had lots of questions on this at work so its a current topic.
In the "old days" the flasher cans fitted to cars was an electromechanical unit that used the lamp current to repeatedly heat and cool a bimetalic strip - making and breaking the circuit generating the flash. Flash rate was VERY dependant on the load current ( number and size of lamps ).
More recently, "electronic" flasher cans are the norm, these use electronice to controll the flash rate and are not DIRECTLY load dependant.
HOWEVER there is a "feature" of these electronic flasher cans that we need to know about - lamp failure detection. These cans are still load sensitive - if a bulb fails they switch to a high blink rate to alert the driver to this condition. Each flasher can has a 'rating' of number and size of lamps.
Given the much lower operating draw of LED systems it is almost certain that fitting LED indicators will trigger the lamp fail detection in flasher cans resulting in a high flash rate. Whilst this is still technically legal ( the fail mode obviously can't be illegal from the OEM ) it will draw unwanted attention to the vehicle.
There are two solutions to the problem - each with pro's and cons....
1/ Fit a load resistor to the system to draw extra current to make up for what isn't being used by the LEDS. Seems a shame to save power and then waste it again but...... This is a relatively simple solution for older vehicles ( like most of ours.... ) but can interfere with electronic systems in modern vehicles as the load is not distributed where the electronics expects to find it. In some cases it can even cause ABS fault codes etc etc..... somewhat surprising but a fact.
Unfortunately as the majority of the load current is now drawn by the extra resistor the lamp failure mode is no longer operative - loose an LED indicator and you won't know about it.
2/ Adjust the load sensitive characteristics of the flasher can. This requires some technical exploration of the actual can in the car but this results in the retention of the lamp failure mode for your LED lamps... a good thing. Once again not advised in modern vehicles as there may not be a flasher can - simply a "sounder" to generate the "click", or indicators may be network driven.
Anyone with modest electronic ability may well be able to identify the components in the can responsible for failure detection, in most cases its a single chip and a few resistors that control the flash rate. I would avoid fitting an adjustable resistor except during development as they aren't very reliable. Set the value and then solder in a fixed resistor. You now have the correct flash rate and failure detection.
So there it is... modern cars - leave well alone unless your absolutely certain of the effects - older cars, issues that can be solved via either approach without ongoing technical problems.
In the "old days" the flasher cans fitted to cars was an electromechanical unit that used the lamp current to repeatedly heat and cool a bimetalic strip - making and breaking the circuit generating the flash. Flash rate was VERY dependant on the load current ( number and size of lamps ).
More recently, "electronic" flasher cans are the norm, these use electronice to controll the flash rate and are not DIRECTLY load dependant.
HOWEVER there is a "feature" of these electronic flasher cans that we need to know about - lamp failure detection. These cans are still load sensitive - if a bulb fails they switch to a high blink rate to alert the driver to this condition. Each flasher can has a 'rating' of number and size of lamps.
Given the much lower operating draw of LED systems it is almost certain that fitting LED indicators will trigger the lamp fail detection in flasher cans resulting in a high flash rate. Whilst this is still technically legal ( the fail mode obviously can't be illegal from the OEM ) it will draw unwanted attention to the vehicle.
There are two solutions to the problem - each with pro's and cons....
1/ Fit a load resistor to the system to draw extra current to make up for what isn't being used by the LEDS. Seems a shame to save power and then waste it again but...... This is a relatively simple solution for older vehicles ( like most of ours.... ) but can interfere with electronic systems in modern vehicles as the load is not distributed where the electronics expects to find it. In some cases it can even cause ABS fault codes etc etc..... somewhat surprising but a fact.
Unfortunately as the majority of the load current is now drawn by the extra resistor the lamp failure mode is no longer operative - loose an LED indicator and you won't know about it.
2/ Adjust the load sensitive characteristics of the flasher can. This requires some technical exploration of the actual can in the car but this results in the retention of the lamp failure mode for your LED lamps... a good thing. Once again not advised in modern vehicles as there may not be a flasher can - simply a "sounder" to generate the "click", or indicators may be network driven.
Anyone with modest electronic ability may well be able to identify the components in the can responsible for failure detection, in most cases its a single chip and a few resistors that control the flash rate. I would avoid fitting an adjustable resistor except during development as they aren't very reliable. Set the value and then solder in a fixed resistor. You now have the correct flash rate and failure detection.
So there it is... modern cars - leave well alone unless your absolutely certain of the effects - older cars, issues that can be solved via either approach without ongoing technical problems.
